A 100- or 300-litre buffer tank: why sizing should not be guesswork

A buffer tank appears alongside the heat pump in the quotation. Asked about its volume, the answer is often: ‘That is what we always install’ or ‘You need a certain number of litres per kilowatt’.

My first concern is different: what specific task does this tank perform, and does it remain active in all the required operating modes?

Heat storage, flow rate and hydraulic separation are different tasks

A buffer tank can reduce short cycling and, with a suitable arrangement, hydraulically separate the heat source and load pumps. But the result depends on the pipework arrangement and controls. Caleffi discusses different connection configurations and how the tank volume participates in the thermal process. (Caleffi — Design Details for Air-to-Water Heat Pump)

Not every tank automatically acts as a hydraulic separator. Nor does every low-loss header contain enough water to store the required energy.

The minimum flow rate and the manufacturer’s requirements for available water volume, including for defrosting, must be checked separately. Meeting one condition does not prove that the others are met.

Excess capacity matters for cycling

Consider an inverter-driven heat pump that cannot reduce its heat output below 6 kW under the operating conditions in question. The open zone needs only 2 kW. The 4 kW difference must be absorbed somewhere while the compressor is running.

For a simplified estimate of the active water volume:

V=\frac(P₍\min₎-Q₍load₎)× t×604,18×Δ T

Here, power is in kW, time in minutes and volume in litres. Water density is taken as 1 kg/l. ΔT is the permissible temperature change in this volume between the start and end of the cycle, not the flow–return temperature difference. This calculation is based on the heat balance also used in Caleffi’s methodology. (Caleffi — Design Details for Air-to-Water Heat Pump)

For an assumed duration of 10 minutes and a temperature change of 5 K:

V=((6-2)×10×60) / (4,18×5)≈115 l

Ten minutes is an assumption in this example, not a universal compressor requirement.

This does not necessarily mean a 115-litre tank

The calculation gives the active volume required for the specified scenario. If 40 litres are already permanently available in the pipework and equipment and participate in the same temperature cycle, the calculated shortfall is around 75 litres.

But water in a zone closed off by a thermal actuator cannot be counted. Nor can the tank’s entire nominal volume automatically be counted if the pipework arrangement leaves some of its water almost stagnant.

An inverter drive does not mean that a buffer tank is never needed. What matters is the relationship between minimum output and the smallest active load; Caleffi’s publications address this case separately. (Caleffi — Key Design Concepts)

A larger tank does not automatically mean a better system

Suppose a 300-litre tank operates over a 5 K temperature range. Its idealised stored thermal energy within this range is only:

E=300×0,001163×5≈1,74 kWh of heat

At a 10 kW load, that is about ten minutes. Such a tank may benefit the unit’s operation, but it should not be sold as a heating reserve lasting several hours.

We would agree the tank’s function, operating temperatures, pipework arrangement, sensor positions and zone closure conditions first. Only then would we determine its volume.

A buffer tank is not sized to match the figure on the outdoor unit. It is sized for the specific excess capacity, operating modes and hydraulics.